Nitric oxide (NO) has been implicated as part of a cascade of interacting agents involved in a wide variety of bioregulatory processes, including the physiological control of blood pressure, macrophage-induced cytostasis and cytotoxicity, and neurotransmission (Moncada et al., “Nitric Oxide from L-Arginine: A Bioregulatory System,” Excerpta Medica, International Congress Series 897, Elsevier Science Publishers B.II.: Amsterdam (1990); Marletta et al., Biofactors 2: 219-225 (1990); Ignarro, Hypertension (Dallas) 16: 477-483 (1990); Kerwin et al., J. Med. Chem. 38: 4343-4362 (1995); and Anggard, Lancet 343: 1199-1206 (1994)). Given that NO plays a role in such a wide variety of bioregulatory processes, great effort has been expended to develop compounds capable of releasing NO. Some of these compounds are capable of releasing NO spontaneously, e.g., by hydrolysis in aqueous media, whereas others are capable of releasing NO upon being metabolized (Lefer et al., Drugs Future 19: 665-672 (1994)).
Keefer et al. (U.S. Pat. Nos. 4,954,526; 5,039,705; 5,155,137; 5,208,233 and 5,405,919 and related patents and patent applications, all of which are incorporated herein by reference) disclose, among others, the use of certain nucleophile/nitric oxide adducts as NO-releasing agents, i.e., in which the nucleophile residue (Nuc) is a primary amine, a secondary amine or a polyamine. Although such adducts offer many advantages over other currently available nitric oxide-releasing compounds, one disadvantage presented by the use of such adducts as pharmaceutical agents is the potential risk of release of nitrosamines, which are carcinogenic, upon decomposition and release of NO. Another disadvantage of the adducts of primary amines is that they can be unstable even as solids due to a tendency to form traces of potentially explosive diazotates.
Several types of compounds of the general structure have been known for many years. See Hrabie and Keefer, Chem. Rev. 102, 1135-1154 (2002) for a review of diazeniumdiolate chemistry. Traube (Liebigs Ann. Chem. 300: 81-123 (1898)) reported the preparation of a number of such compounds and noted that treatment of the compounds with acid produced a “brown gas.” Although brown gas suggests the release of NO, given that a brown gas also may be produced in the disproportionation of nitrite, the release of brown gas by the compounds prepared by Traube is not, in and of itself, evidence of NO release. Compounds of the structural type reported by Traube are known to require harsh treatment with mineral acids to release any gas, which is incompatible with a biological utility.
Another compound, named cupferron, of the structure has been shown by Kubrina et al., Izvestia Akademii Nauk SSSR Seriia Biologicheskaia 6: 844-850 (1988)) to generate NO in vivo. In addition, the antibiotics alanosine (C(O)(OH)CH(NH2)CH2 N(O)═NOH) and dopastin (CH3CH═CHC(O)NHCH2 CH(i-propyl)-N(O)═NOH), as well as cupferron, have been shown to release NO in vivo by enzymatic oxidation (Alston et al., J. Biol. Chem. 260: 4069-4074 (1985)).
More recently, Keefer et al., in U.S. Pat. No. 5,212,204, broadly described that an organic moiety may be linked via a carbon to the N2O2− group. This patent does not disclose an amidine structure as the nucleophile, nor does it teach the nature of the structural characteristics that an organic moiety must possess such that the resulting N2O2− group is a nitric oxide donor.
In this regard, a recent study of the N2O2− group (Taylor et al., J. Org. Chem. 60: 435-444 (1995)) proposed a mechanism for the observed NO release. The proposed mechanism was based on quantum mechanical calculations, which showed protonation at the terminal oxygen to be most favored thermodynamically in the case of N bound N2O2−.
None of the above disclosures, however, mention anything about the release of nitroxyl (HNO, which, at the physiological pH of 7.4, exists as NO−) by this functional group. Recent results suggest that, under certain conditions, many classes of “NO donors” may release some NO− (see the discussions for nitrosothiols and diazeniumdiolates as well as the table of NO donors in Feelisch et al., Donors of Nitrogen Oxides, Methods in Nitric Oxide Research, M. Feelisch and J. S. Stamler, Eds., Ch. 7, pp. 71-115, John Wiley and Sons, New York (1996)).
To date, there are three compounds used to generate HNO in solution. One compound, Angeli's salt, which is the standard HNO source (Fukuto et al., J. Pharm. Exp. Ther. 263: 546-551 (1992)), is an inorganic salt. The other two compounds, acetylated Piloty's acid (Smith et al., J. Amer. Chem. Soc. 82: 5731-5740 (1960)) and benzoylated hydroxycyanamide (Lee et al., J. Med. Chem. 35 3648-3652 (1992)) are promising inhibitors of aldehyde dehydrogenase. However, even in these compounds, there is debate as to whether the observed physiological effects are attributed to NO, or to NO−. For example, Piloty's acid has been shown to release NO oxidatively under physiological conditions (Zamora et al., Biochem. J. 312: 333-339 (1995)).
Reports that superoxide dismutase can prolong the effects of NO via its reversible reduction to NO− (Murphy et al., PNAS USA 88: 10860-10864 (1991)) and that NO−, itself, exhibits potent activity as a vasodilator (Fukuto et al., J. Pharm. Exp. Ther. 263: 546-551 (1992)) and as an inhibitor of aldehyde dehydrogenase (Lee et al., J. Med. Chem. 35: 3648-3652 (1992)) suggest that compounds, which release either NO or NO- or mixtures of the two, are potentially useful pharmaceutical agents and may even offer advantages over compounds that just release NO.
Despite the extensive literature available on NO and nitric oxide-releasing compounds, there remains a need for stable nitric oxide-releasing compounds in which the nitric oxide-releasing group N2O2− is bonded directly to a carbon atom and which can be prepared from compounds that do not include a nitrogen atom suitable for conversion to a diazeniumdiolate. Moreover, because not all compounds, such as proteins, are stable to direct treatment with nitric oxide, there is a need for other synthetic routes that enable the production of a previously inaccessible class of compounds. These and other objects of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.